A Lorenz microscope method has been developed as a method of observing a behavior of deflecting an electron beam transmitting a magnetic material by receiving a Lorenz force by magnetizing a sample as its name signifies. However, currently, the method is received as a method of visualizing a deflection state of an electron beam, or a method of visualizing an electron beam receiving a deflection by an interactive operation different from Bragg diffraction caused by a crystal structure not only for the magnetic material but a dielectric polarization, a strain field or the like. Roughly classified, there are two methods of a Foucault method and a Fresnel method in the Lorenz method (Nonpatent Literature 1), in term of the magnetic material, the Fresnel method is a method of observing a domain wall, and the Foucault method is a method of observing a magnetic domain.
In the following, an explanation will be given of respective methods of the Fresnel method and the Foucault method by taking an example of observing the magnetic material having a 180-degree inversion magnetic domain structure. Further, a simple description will be given to electron beam holography, an intensity transportation equation method, and a small angle diffraction method as examples of other method of methods of visualizing a small deflection electron beam using a transmission electron microscope.
<Fresnel Method>
FIG. 1 shows a behavior of an electron beam receiving a deflection by a magnetic material having a 180-degree inversion magnetic domain structure. An angle of deflecting an electron beam depends on a magnitude of a magnetization and a thickness of a sample. Therefore, in a case of a sample having a constant thickness and a uniform magnetization, the angle of deflection received by the electron beam stays the same in any domain, and an azimuth and a direction differ in accordance with a magnetic domain structure. As shown in FIG. 1, when electron beams 27 are incident on a sample 3 having the 180-degree inversion magnetic domain structure, the electron beams 27 transmitting the sample 3 receive deflections in inverse directions by the respective magnetic domains (31, 33). When the electron beams 27 receiving the deflections are propagated by sufficient distances below the sample, there are generated a situation of overlapping each other and a situation of separating from each other inversely on a projected face 24 at positions in correspondence with 180° magnetic walls 32. The Fresnel method focuses condensation and rarefaction of an intensity of the electron beam on the projected face 24. A graph 25 of an intensity distribution of an electron beam on the projected face is exemplified at a lower portion of FIG. 1.
FIG. 2 is a schematic view of an optical system when a magnetic material is observed by the Fresnel method. A Fresnel image 86 is exemplified at a lower portion of FIG. 2. FIG. 2A shows a behavior of observing by focusing not a sample but a space position 35 on a lower side of the sample, and a portion of exactly the magnetic wall 32 is observed by a contrast 72 of a bright line (white color) or a dark line (black color). Similarly, as shown in FIG. 2B, even when a space position 36 on an upper side of the sample is focused, a portion of the magnetic wall 32 is observed by an inverse contrast 72. That is, a boundary line of a domain giving a deflection to an electron beam is observed by a bright line (white color) or a dark line (black color) by observing the sample by defocusing the sample. A white and black contrast of the boundary line of the Fresnel image at this occasion depends on a combination of the deflection direction and a position of focusing. Also, an amount of defocusing (defocusing amount) depends on a magnitude of a deflection received by an electron beam, and although in a case of a large deflection, a sufficient contrast is obtained by a small defocusing amount of about several hundreds nm, in a case of an observation object giving only a small deflection as in, for example, a fluxoid quantum, a defocusing amount of several hundreds nm is needed.
<Foucault Method>
FIG. 3 shows an optical system of observing a magnetic domain structure by a Foucault method. Similarly to FIG. 1, electron beams transmitting the sample 3 having the 180-degree inversion magnetic domain structure receive deflections in directions inverse to each other by the respective magnetic domains (31, 33), and the electron beams receiving the deflections in the directions are spotted (11, 13) at positions in accordance with deflection angles thereof at, for example, a rear focal point face 54 of an objective lens 5 (strictly speaking, an image face of a light source by the objective lens). Hence, an objective aperture 55 is inserted and only an electron beam transmitting a magnetic field intended to observe is selected and focused on an image face 7. For example, in FIG. 3A, this is an example of selecting an electron beam transmitting through the magnetic domain 31 and deflected in an upper left direction of paper face, and FIG. 3B shows an example of selecting an electron beam inversely transmitting through the magnetic domain 33 and deflected in an upper right direction on paper face. At any rate, a magnetic domain which is selected is observed in white color, and a magnetic domain which is not selected is observed in black color (electron beam does not come), and in a case of a 180-degree inversion magnetic domain structure, the respective magnetic domains (31, 33) are visualized as Foucault images 84 in stripe shapes (71, 73).
Although in the Foucault method, the sample image is observed in focus, and therefore, a high resolution observation is expected, for example, in a case of a magnetic material, the deflection angle of the electron beam is as small as about 1/10 of a Bragg angle by the crystalline sample, and therefore, an objective aperture having a small aperture diameter needs to be used, and an obtained spatial resolution is about 1/10 times as much as a lattice resolution, which is not significantly different from that in the Fresnel method. Further, an origin of a contrast for observing the magnetic domain structure is caused by shielding an electron beam which is transmitted through a magnetic field which is not observed, and this has been a method of obtaining the contrast by abandoning a portion of information. Therefore, for example, in a case of observing an object extending plural magnetic domains as in crystal drain boundaries, it is necessary to readjust the objective aperture and separately observe the Foucault image having an inverse contrast, or an ordinary electron microscope image needs to be observed additionally by deviating the objective aperture from an optical axis. That is, observations at plural times are needed, and a dynamic observation, a real time observation or the like has substantially been impossible.
As one of methods of dealing with a defect by the Foucault method described above, although an illustration is omitted, there is proposed a method of further deflecting the propagation angle of the electron beam receiving the deflection by the sample by using an electron beam biprism in an irradiating optical system, and observing and recording plural Foucault images once by focusing the images at locations different from each other on the observation face (twin Foucault method) (Patent Literature 1) (Nonpatent Literature 2). The method proposes a new concept of one mirror two images (information), in implementing the Foucault method, conditions to be added to a conventional Lorenz electron microscope of not only the magnetic shielding lens but the electron beam biprism or the like are increased in implementing the Foucault method. Therefore, it seems that a small period of time is taken for spreading the method.
<Lens-Less Foucault Method>
In recent times, there has been developed a method of enabling to implement the Foucault method and the small angle electron diffraction by using an ordinary general-use transmission electron microscope which does not include a magnetic shielding lens (Nonpatent Literature 3). This is the lens-less Foucault method. The “lens-less” described here signifies that an objective lens is turned off and is not used for focusing. A description will be given later of details of the method. (Incidentally, the present invention has been carried out for executing an effective experiment by alleviating a burden of an electron microscope operator in operating the device in implementing the lens-less Foucault method, and relates to a control of an optical system by the lens-less Foucault method.)
<Other Lorenz Method>
Other than the Lorenz microscope method described above, there have been developed electron beam holography (Nonpatent Literature 4), an intensity transportation equation method (Nonpatent Literature 5) and the like as methods of observing the magnetic domain structure of the sample from a phase distribution of an electron beam. Although any of the methods have respective advantages, it is an actual situation that there are a number of complications in implementing the methods such that an electron beam having a high coherence of an electric field emission type electron beam is needed, an electron beam biprism is needed as an additional device in the electron beam holography, a domain for transmitting a reference wave needs to be considered in a sample shape, at least 2 sheets of images defocusing amounts of which are already known are needed by interposing an image in focus (a total of three sheets of images) in the intensity transportation equation method, and magnifications and an adjusting process of positioning or the like for respective images are indispensable and so on.
<Small Angle Diffraction Method>
In recent times, the method of observing a deflection angle of the electron beam by a magnetization in a sample as a diffraction spot at a diffraction face has begun to be implemented (Nonpatent Literature 6). The method is a method of observing the small diffraction angle of the electron beam as a diffraction pattern at a diffraction face (that is, as a diffraction pattern of a large camera length), which was implemented in 1960 year generation (Nonpatent Literature 7), and this is a technology which has been forgotten for a long time thereafter. This is a method which is effective for obtaining information of an average deflection angle, which is reconsidered as a method of detecting a deflection angle of a transmission electron beam receiving from an entire irradiating area of the electron beam which is an average value rather than detecting a small deflection angle of a transmission electron by an individual element when the deflection angle of the electron beam is reduced by miniaturizing and thinning a magnetic element.
Table 1 summarizes a main observation object, a deflection angle received by an electron beam of an acceleration voltage 300 kV, and a camera length needed for observation.
TABLE 1Deflection angle received by 300 kV electron beam and cameralength needed for observationDeflection angle ofCamera length neededObservation objectelectron beam (rad)for observation (m)Crystal (Bragg10−2100diffraction)Long period structure10−3101Magnetic body (magnetic10−4 through 10−5102 through 103domain)Dielectric substance10−5 through 10−6103 through 104(dielectric polarization)Metal superconductor10−6 through 10−7104 through 105fluxoid quantumHigh temperature10−7 or less105 or moresuperconductor fluxoidquantum<Foucault Method and Small Angle Diffraction Method>
As described above, it is necessary for optimally implementing the Foucault method to pertinently use an angle restricting aperture at a diffraction face. For example, in a case of a magnetic material (magnetization 1 T (tesla)) having a thickness of 50 nm which can be easily transmitted by an electron beam of an acceleration voltage 300 kV, a deflection angle by a magnetism becomes about 2×10−5 rad, which is an angle nearly 1,000 times smaller than a Bragg diffraction angle by crystal. Therefore, in the Foucault method, it is necessary to be able to realize the small angle diffraction method as a matter of fact for improving a focusing accuracy thereof. That is, it is necessary to construct an optical system such that a diffraction pattern having a large camera length in correspondence with the small angle diffraction is formed (it is necessary to construct an optical system which can magnify the diffraction pattern). Moreover, it is necessary that a diffraction face of a diffraction pattern and a face of inserting an angle restriction aperture coincide with each other.
Although an explanation has been given of the Lorenz method as a magnetic material observing method by a transmission electron microscope as described above, the observation object is not limited to the magnetic material as described above. Above all, in a view point of visualizing or focusing an electron beam having a small deflection angle, the method has a technical side view which is common also to a phase difference electron microscope method for a biological sample, an organic sample or the like.